In the early history of microphones there were only vacuum tube based amplifiers accessible. The high output impedance of the vacuum tubes, together with the high voltage of some 100 Volts DC needed to drive the vacuum tubes, forced engineers to use a transformer based coupling to drive the low-impedance input of a typical mixer table.
Conventionally, microphone assemblies include a microphone element and a pre-amplifier circuit in a fixed configuration within a common housing. The microphone assembly typically also includes an output terminal in which the signal generated by the microphone element and amplified by the microphone pre-amplifier circuit is output. The power needed to drive the vacuum tube microphone pre-amplifier circuits were so high, that a separate power unit had to be used to drive the vacuum tube microphone pre-amplifier circuit. The vacuum tube microphone pre-amplifier circuit is normally built-in the microphone housing. In order to send a rather noise sensitive signal generated by the microphone, over lengthy wires, is used a common transfer technique. This known transfer technique uses two signal wires and a common ground wire. The two signal wires contain, in principle, the same signal, at least amplitude wise. The first signal wire contains a signal which is phase shifted by 180 degrees and the second signal wire contains an un-shifted signal, both signals having the same amplitude. Hence the difference of the both signals may be calculated as S−(−S)=2S. Any noise entered via the wires will remain ‘un-shifted’ and theoretically the noise will, in the subtraction stage, disappear. This is proven by the calculation of N−N=0.
Next step in the evolution was the introduction of the transistor. A transistor based microphone pre-amplifier consumes a very small amount of power compared to a vacuum-tube based microphone pre-amplifier, thus eliminating the need for a separate power supply to the combined microphone pre-amplifier circuit and microphone device. The transistor microphone pre-amplifier circuit in known microphone assemblies is arranged such that the microphone pre-amplifier has two outputs (‘hot’ and ‘cold’). A first output is driven with a 180 degree phase shift compared to a second output. Thereby emulating the ‘old’ signal properties and enabling the already known solution on signal to noise reduction by utilization of differential phase-shifted signals. Modern high-end microphones still today sometimes utilize a transformer, having a common mid-point outlet used as signal ground, to drive the ‘hot’ and ‘cold’ output. The transformer is a heavy and expensive component, thus a ‘transformer-less technology’, built on transistor technology was introduced some decades ago.
An input stage of known microphone pre-amplifier circuits, based on transistor technology, normally has high input impedance in order to prevent signal reduction. The pre-amplified signal from the microphone pre-amplifier circuit is normally output to an audio amplifier, or a mixer table, which provides the major signal gain and mixing possibilities. Prior art microphone pre-amplifier circuits for capacitive transducers are low output impedance voltage output devices and a problem with these prior art microphone pre-amplifier circuits is the need for expensive tuning of circuits, i.e., a circuit may have to be laser tuned in order to meet the demands of precision. Thus the ‘transformer-less technology’ has a disadvantage, namely the transistor microphone pre-amplifier circuit has to be configured such that it has a very exact precision in both amplitude and phase shift. This precision, as indicated by audio expertise, has to be better than ±1%. This may off course be achieved by present circuits, but requires serious and very expensive laser tuning of circuits.
A further problem with existing solutions is the lack of possibility to ‘squeeze’ out as much amplification as one would want to. Should the known microphone pre-amplifier circuits be set to maximum amplification it would be at the risk of instability in the feed-back loop. The feed-back loop includes input of the so called ‘phantom’ power supply and microphone pre-amplifier low impedance output. The ‘phantom’ power supply is when there is no need for a separate power supply to the microphone pre-amplifier circuit and the microphone. Yet a further problem of known solutions is the need for an output coupling capacitor of the microphone pre-amplifier circuit. Without the output coupling capacitor a current rush would be the inevitable result of connecting a DC power supply to the microphone pre-amplifier circuit. This can be concluded since known microphone pre-amplifier circuits are low output impedance circuits. As a result this would lead to an output circuit of the microphone pre-amplifier circuit that, due to its low output impedance and the fact that its working-point normally is around the middle of the supplied voltage (typically 48 VDC), would suffer from a current rush caused by half the supply voltage. The supply voltage has to be at this level in order to make the microphone pre-amplifier circuit to work properly. As stated above, half of the supplied DC voltage would cause a large current to rush through the, rather low, output impedance causing severe damage to the microphone pre-amplifier circuit. Yet another problem with known solution is the size of the microphone pre-amplifier circuit. In a combined microphone and microphone pre-amplifier assembly it is vital to keep the size down at a minimum in order to fit both microphone element and pre-amplifier circuit in a common housing. As can be concluded by the discussion above, there is a need for improved microphone pre-amplifier circuit.